RP118

CORRECTING ENGINE TESTS FOR HUMIDITY

By Donald B. Brooks

ABSTRACT

Data obtained on a 6-cylinder automobile engine indicate a loss of engine power

with increasing humidity proportional to the volumetric loss of oxygen content

of the atmosphere. It is shown that power and fuel consumption may be cor-

rected by subtracting observed water vapor pressure from atmospheric pressure

and using the result in place of barometric pressure in the usual correction

formula. The humidity correction may be as large as that due to changes in

barometric pressure.

Simple nomograms are presented for obtaining the humidity correction, both

near sea level and at higher altitudes. An appendix gives methods of computation

of these nomograms.

CONTENTSPage

I. Introduction 795

II. Test apparatus and procedure 795

1. Test series No. 1 797

2. Test series No. 2 797

3. Test series No. 3 798

III. Discussion of results 799

IV. Humidity correction chart 801

V. Conclusions 803

VI. Appendix—Method of computation of charts 803

I. INTRODUCTION

Tests made by A. W. Gardiner x using a 1-cylinder engine having

indicated that atmospheric humidity has a very appreciable effect onsome phases of engine performance, a test program was undertaken

at the Bureau of Standards further to study this effect, using a

multicylinder engine.

II. TEST APPARATUS AND PROCEDURE

Tests were made on a 6-cylinder, 3-port, overhead valve engine

of 3%-inch bore and 4%-inch stroke, coupled to a Sprague electric

dynamometer and spark accelerometer. In the three series of tests

three different fuels were used, being selected so as to give little or

no detonation at optimum spark advance under any test condition.

Power measurements were made on the dynamometer and friction

measurements by use of the spark accelerometer,2 the latter being

iSee J. S. A. E., p. 155; February, 1929.

2 Method described in paper by Brooks on Operating Factors and Engine Acceleration presented at

S. A. E. annual meeting, January, 1929. See J. Soc. Automotive Eng., p. 130; August, 1929.

795

796 Bureau of Standards Journal of Research [Vol. s

checked against friction measurements on the dynamometer. Humid-ification was obtained by passing steam and cold air into a mixing

chamber and thence to an air heater. Measurements of humidity

were made by continuously passing a part of the carburetor air supply

over calibrated dry and wet bulb thermometers graduated to 0.2° F.

Measurements of humidity are expressed as pressure of water vapor

in mm Hg.

Figure 1.

—

Effect of humidity on engine performance

Tests were made at full throttle at an engine speed of 500 r. p. m.

Cylinder and manifold jackets were maintained at the same tem-

perature, this being from 60° to 80° C. in the different series of tests

but being constant for any one series.

In the first two series of tests readings were taken at from 6 to 8

spark advances for each humidity and air-fuel ratio. From the

results, plotted against spark advance, faired values of maximumpower and optimum spark advance were obtained. In the third

test series optimum advance was found by trial.

Brooks] Correcting Engine Tests for Humidity

1. TEST SERIES NO. 1

797

The tests of this series were made with a mixture of 2 parts of

eastern domestic aviation gasoline to 1 part of motor benzol. Afixed carburetor adjustment was used, giving an air-fuel ratio of

about 13.5. Carburetor air temperature was maintained at 30° C.

Optimum power and spark advance were determined at humidities

from 5.1 mm Hg to saturation (31.9 mm Hg). Figure 1 shows the

results, plotted against water vapor pressure.

Figure 2.

—

Effect of humidity on power

2. TEST SERIES NO. 2

The tests of this series were made with a mixture of equal parts of

eastern domestic aviation gasoline and motor benzol. A series of

5 carburetor metering jets were used, giving air-fuel ratios from about

12 to about 16. Carburetor air temperature was maintained at

30° C.

With each air-fuel ratio optimum power and spark advance were

determined at two humidities, 4.5 and 27 mm Hg, respectively.

Figure 2 shows the results, plotted against water-vapor pressure.

It is notable that with orifice 43 an apparent increase of power with

798 Bureau of Standards Journal of Research [Vols

humidity is shown. This is the leanest orifice used; the apparent

increase in power seems to be due to automatic enrichment of the

mixture at higher humidities.

3. TEST SERIES NO. 3

The tests of this series were made with a commercial brand of

aviation gasoline approximately equal in antiknock value to a mixture

of equal parts of eastern domestic aviation gasoline and motor benzol.

Figure 3.

—

Effect of humidity on power

For this series of tests the carburetor was equipped with a needle

valve, and tests were made over a range corresponding roughly to

air-fuel ratios of 9 to 17. Carburetor air temperature was main-

tained at 41° C.

At two humidities, corresponding to 13.4 and 58.2 mm Hg, fuel

consumption, power, and optimum spark advance readings were

taken at 12 points over the range of air-fuel ratios stated above.

Results are shown in Figures 3 and 4.

Brooks] Correcting Engine Tests for Humidity

III. DISCUSSION OF RESULTS

799

The tests shown in Figure 1 indicate a linear relation between loss

of power and absolute humidity; the more extensive tests by Gardiner

agree with this. Moreover, if the humidity be expressed as per-

centage of barometric pressure, the loss of power in percentage is

roughly equal to the humidity. From this has arisen the "oxygen

content" hypothesis, stating that the power is proportional to the

oxygen content of unit volume of the atmosphere.

gfftt£-j"t!Hfl#ill

1

1

1

1

mumItttttifHi |ggfggfgg3ffl 1 illilltHITTI

'j:rn:g

1

-Jf- f^

^^ilx^rr rrwrijfejiffi^S

iffljjjjiji| t

±F.:H:§

itfirr.l.ll

.. ._

Figure 4.

—

Effect of humidity on specific fuel consumption

To test this hypothesis, values of loss of maximum power from the

three series of tests were plotted against the loss predicted on the basis

of the oxygen content hypothesis. Figure 5 shows the agreement be-

tween the measurements and the hypothesis, the weighted mean ob-

served loss of power being 101 per cent of that predicted. However,

other factors than decrease in oxygen content may affect the power.

Figure 6 summarizes the results in regard to variation of optimumspark advance with humidity. A decided increase in spark advance

is seen to be required with increasing humidity. This rate of increase

seems to be a constant, irrespective of the magnitude of advance.

800 Bureau of Standards Journal of Research [Vol. S

The upper curve is the mean of observations by Gardiner on another

engine, operating at different speed and compression ratio and withgenerally different operating conditions. For all these curves, how-ever, the required advance is 2.1° per cm Hg of water-vapor pressure

within the limits of experimental error. On the basis of curves pre-

sented in N. A. C. A. Technical Eeport No. 276, and if the progress

of combustion is similar at all humidities, this rate of increase of spark

advance should entail a loss of power equal to 13 per cent of that due

to the decrease of oxygen; that is, if only oxygen content and spark

advance affect the power, the loss of power should be 113 per cent of

that predicted on the basis of the "oxygen content " hypothesis.

Figure 5.

—

Summary of tests showing effect of humidity on power

On the basis of the Bureau of Standards tests, which show but 101

per cent ±2.6 per cent of the loss predicted from the oxygen content

hypothesis, there is a 99.8 per cent probability that other factors tend

to compensate for the loss occasioned by reduction of oxygen and in-

crease of optimum spark advance. Such other factors may include

lower radiation, dissociation, and less change of specific heats, due to

lower maximum temperatures.

In Figure 4 it is seen that the specific fuel consumption curves at

the two humidities are displaced horizontally but have practically the

Brooks] Correcting Engine Tests for Humidity 801

same minimum. Moreover, this horizontal displacement is equal, in

per cent, to the percentage difference in oxygen content. This in-

dicates that fuel consumption as well as power should be corrected

for change in humidity, since fuel consumption is used in place of air-

fuel ratio. This has been done for test series No. 3, in Figure 7.

The results obtained at the two humidity values are seen to He on the

same curve within experimental error.

Figure 6.

—

Effect of humidity on optimum spark advance

IV. HUMIDITY CORRECTION CHART

Figure 8 is a nomogram for obtaining water-vapor pressure (humid-

ity correction to barometer) from wet and dry bulb and barometer

readings. Figure 8 is constructed for units of °C. and mm Hg. Fig-

ure 9 is a similar nomogram for units of °F. and inches Hg.To use these charts, place a straightedge so that it intersects the

t—f scale at the value of the difference between wet and dry bulb read-

ings and intersects the t' scale at the value of the wet-bulb temperature.

At its point of intersection of the true (corrected) barometer value

read the humidity in the units shown on the scale at the extreme right.

For convenience a barometer-temperature correction nomogramis located at the lower right of the chart. To use this, align a straight-

802 Bureau of Standards Journal of Research [Vol. 8

edge through the center of the small circle at the bottom of the chart

and through the barometer temperature on the vertical scale to the

right. At its intersection with the observed barometer reading read

barometer correction on the same scale used for humidity correction.

This correction chart is for barometers with brass scales.

Figure 7.

—

Verification of humidity correction to power and

fuel consumption

The humidity charts are based on Smithsonian values 3 for water-

vapor pressure and on the formula deduced by Professor Ferrel 4

>/ - 0.00036750- o(l +Y5^)for English units in which

6 = pressure of water vapor in inches Hg corresponding to dry

and wet bulb temperatures / and t' in °F., respectively.

B= true barometric pressure in Hg.

e' = saturation water-vapor pressure at f,

and on the same formula with appropriate constants for metric units.

8 Smithsonian Meteorological Tables.

i Annual Report of the Chief Signal Officer, Appendix 24, pp. 233-259; 1886.

t = DRY BULB

t' = WET BULB

UNITS —°C 4 MM HG.

°1-lOX -2

70Q 810

760

Figure 8.

—

Nomogram for obtaining for humidity and barometer temperature

t = DRY BULB

t'= WET BULB

UNITS -"FA IN. HG.

-I20fU100

h 80-

§60-00 40-

Figtfre 9.

—

Nomogram for obtaining for humidity and barometer temperature

Brooks] Correcting Engine Tests for Humidity 803

It is to be noted that these charts assemble barometer corrections

significant in automotive work on one sheet, are sufficiently precise

for their purpose, and are less laborious and less productive of errors

of computation than psychrometrie tables or contour charts. Other

barometer corrections include free-air altitude, latitude, and capil-

larity. The first two of these total less than 1 mm, while the latter

is of the opposite sign and of much the same magnitude; hence, these

three corrections are negligible for automotive work in this country.

In correcting engine-performance data to standard conditions cor-

rections for both humidity and barometer temperature are to be

subtracted from the observed barometer reading to give air pressure.

Observed power and corresponding fuel flow are then multiplied bythe pressure correction factor (standard pressure/air pressure), thus

allowing for variations in atmospheric pressure and humidity.

V. CONCLUSIONS

1. This work shows definitely that failure to allow for the effect of

differences in atmospheric humidity may introduce errors as great as

would be occasioned by failure to allow for changes in barometric

pressure. Under extreme conditions either correction may amountto nearly 10 per cent of the indicated power.

2. Under all atmospheric conditions normally encountered in

automotive testing, humidity may be allowed for by deducting the

observed pressure of water vapor from the barometric pressure used

in the power computations.

3. Due to cancellation of opposing factors the proposed correction

represents the observed effect of humidity well within the usual pre-

cision of power measurements.

4. In correcting engine-performance data at different air-fuel ratios

the fuel flow values must be multiplied by the same coefficient as the

power values.

5. Optimum spark advance increases linearly with increasing

humidity.

6. Charts are presented for the convenient determination of

humidity values.

VI. APPENDIX—METHOD OF COMPUTATION OF CHARTS

The Ferrel formula for computation of absolute humidity viz,

€ = ^-0.000367^1 +yZ^y)(*-0 (1)

reduces, for a selected value of B, to

e = e'-{a+ W) (t-f) (2)

where a and o are constants derived from the Ferrel formula, e' is

the water vapor pressure at f , and e the absolute humidity at t, V

.

804 Bureau of Standards Journal of Research [Vol. 3

With this as a basis, the chart is constructed as follows: Suitable

scales are selected for (t— f) and for (e), as in Figure 10. Let the

length corresponding to one unit of (t—f) be m; the vertical length

corresponding to one unit of (e) be n; and the horizontal distance

between the '(t— f) and (e) scales be p. The f scale is then located

by the following considerations: When (t-f) is 0, the vapor pres-

sure is obviously e', the saturation pressure at f. When (t— f) has

any value, the vapor pressure is

e1= e1

/ -(a + bt1

f

) ft-V) (2a)

Brooks] Correcting Engine Tests jor Humidity 805

If a line be drawn from on the (t— tf) scale to e' on the (e) scale,

and another line from any other value on the (t—tf) scale to the cor-

responding value on the (e) scale as given by (2a), the intersection of

these lines fixes the corresponding value of tx' on the (tf) scale. In

terms of the scale divisions, the equations of these lines are

neiv——

and

(3)

The solution for the point of intersection gives

pmx=m+ na+ nbtf

(5)

_ mney m + na+ nbtf

where subscripts have been dropped, as the solution is general, giving

the locus of the tf scale in terms of functions of tf. It is to be noted

that the solution for x and y does not contain ft— //); hence, the

requirements of equation (2) are satisfied by a lme. This verifies the

choice of the nomogram. From specific values of x and y from (5)

the tf scale is constructed.

In subsequently constructing scales for values at different baro-

metric pressures the following considerations apply. Since x and yare now to be regarded as fixed, it is desired to alter p so that equation

(1) shall be satisfied at some other barometric pressure. "Calling the

new barometric pressure 7tB, and letting

Pi = value of p with B barometric pressure,

p2 = value of p with JiB barometric pressure,

then, from (3),

Pi V2where

nx= value of n with B barometric pressure,

n^. = value of n with TiB barometric pressure.

Since x also is to be fixed, from (5)

Pim p2m

(6)

m+ ni(a+btf) m+ njiia+btf)(7)

ALTITUDE, FT.

t - DRY BULB

t'= WET BULB

UNITS-C 8, MM. HG.

Figure 11.

—

Novwgram for determiniri; JnntiiiUlii at l,i(/l< nllilaihs

806 Bureau of Standards Journal of Research ivoi. $

Hence,

mpi + n2hapt + njibt'pi— mp2~ nxap2— n1bt'p2 —

^ (Pi—p2) ==n1ap2 (l — h) + n1bt/

p2 (1 — h)

m (j?i—p2) = n1p2 (a+W) (l — h)

p1—p2 :=n1 (a+bt')(l — h)

p2 m

Pi_n1(a+U r

) (l— h) +m

p2

~~m

_ mpi ,s,

^2~m+ 7i1 (l-W(a+ M /

)w

which defines p2 , and hence n2 in terms of known quantities.

Figure 11 is a chart for determining humidity in connection with

high altitude tests, constructed on the basis of (8). From this chart

it is seen that p2 is sensibly constant with t'. Figures 8 and 9 are

based on the Smithsonian Tables, in which p2 can be found from the

relation

p2 =p l -Tc (1-h) (9)

in which ~k is a constant for values of h near unity.

Washington, April 20, 1929.